In marine seismic exploration, a vessel tows acoustic sources and one or more hydrophones arrayed in a streamer. Various heading sensors are also included, generally within or associated with one or more cable levelers. Each of the devices is connected to but external to the streamer through a magnetic coil type coupling, which allows free rotation of the external devices and waterproof connections.
The streamer, along with strength members, hydrophones, couplings, and other components, includes a high data rate conductor for carrying seismic and telemetry data. This high data rate conductor is generally connected in lengths of about 150 meters by a series of modules, which among other functions ensures the amplitude and fidelity of the data. The streamer also includes a low data rate communications conductor, referred to herein as a twisted pair bus, for command and control of the various external devices. As used herein, the twisted pair bus may also be referred to as the control conductor. Magnetic coils for communicating with external devices are connected directly to this bus.
The modules previously referred to serve as repeaters and amplifiers for the high data rate seismic and telemetry data being carried by the high data rate conductor. The twisted pair bus also passes through each of the modules, but in known systems the twisted pair bus has no connections to any other components within each module through which it passes.
This system of streamer high and low speed data conductors has proved effective in conducting seismic surveys but it suffers a number of drawbacks. For example, since the twisted pair bus is one long continuous conductor from the vessel to the end of the streamer, external devices coupled to the streamer must be communicated with in series. This significantly slows the process of querying and commanding the external devices. Also, if one of the external devices develops a fault, such as a short circuit, then the entire external communications system is generally out of commission and the streamer must be retrieved to isolate the fault, and then fault isolation can be a tedious undertaking.
The long, relatively low speed communications lines in the streamer, due to their length, limit the data bandwidth. Furthermore, as a signal is conducted along this communication channel, which is often thousands of meters long, signal strength decreases. The length of the twisted pair bus also has a direct, detrimental effect of the useful life of the charge of batteries in some of the external devices, because they must put out a high power signal in order to be able to communicate from the very end of the streamer.
Sea water in-leakage is also an all too common problem with seismic streamers as they are towed behind a vessel. Minor damage to the skin around the streamer components may develop leaks which permit sea water to invade the streamer when it is towed at depth. Even a minor electrical ground in a portion of the twisted pair bus will ground the entire length of the bus, since the bus is one continuous conductor in known systems. This characteristic also makes it very difficult to pinpoint the location of such a fault in the streamer for diagnostics and repair.
Thus, there remains a need for a way to isolate each of the external devices connected to a streamer so that each can he queried and commanded in parallel, and so that a fault in an external device can be isolated from the rest of the streamer.